There are many useful techniques for fermenting biomass to produce an ethanol-rich biomass. This is commonly referred to as “solid-state fermentation”.
U.S. Pat. No. 4,490,469 describes a method for production of ethanol by fermentation. This patent teaches crushing or pulping biomass to a pulp size less than 10 mm, optionally saccharifying the pulp with acid or enzymes, mixing a yeast suspension with the pulp, waiting for fermentation to complete, and then separating ethanol by pressing or squeezing the liquid from the pulp to produce an ethanol-rich liquid.
U.S. Pat. No. 9,428,772 describes methods and systems for producing fermentation products from carbohydrate-rich substrates. This patent teaches infusing hydrolysis catalysts and fermentation organisms into lignocellulosic biomass using vacuum cycles, waiting for fermentation to complete, and separating ethanol by vacuum stripping.
U.S. Pat. No. 9,499,839 describes methods for fermenting carbohydrate-rich crops. This patent teaches infusing fermentation organisms into sugar-rich biomass, draining excess liquid from the biomass, waiting for fermentation to complete, and separating ethanol by either vacuum stripping or crushing.
U.S. Pat. No. 9,631,209 describes methods for fermenting stalks of the Poaceae family. This patent teaches crushing stalks between rollers while submerged in water containing yeast, draining excess liquid from the stalks, waiting for fermentation to complete, and separating ethanol by either vacuum stripping or crushing.
These and many other solid-state fermentation methods for producing ethanol all have the disadvantage that crushing, pressing, or squeezing generally recovers only about 50% of the ethanol from the biomass, and vacuum stripping is not very useful because the ethanol produced generally contains only about 30% to 40% alcohol by volume (ABV).
While there are many countries where there is a robust market for potable ethanol at 30% to 40% ABV (e.g., cachaca in Brazil, country liquor in India, baijiu in China, and vodka in Russia), there is an even bigger market for using ethanol in generators, in internal combustion engines, and for cooking.
Those skilled in the art will recognize that ethanol at 75% ABV is the minimum concentration required to fire a boiler. Ethanol at 85% ABV is generally the minimum concentration required to run a generator or an internal combustion engine. A fuel-injection system requires at least 92.5% ABV and preferably 96% ethanol ABV. While ethanol at 50% ABV will ignite, ethanol at 60% to 65% ABV is the minimum concentration necessary to support a stable flame for cooking, and ethanol at 80% ABV is necessary to obtain a robust flame for cooking.
The most widely used methods of producing ethanol concentrations above 80% ABV employ distillation columns. This generally starts with a liquid containing 5 vol % to 40 vol % ethanol, boiling it either at atmospheric pressure or reduced pressure, and using a distillation column to produce higher concentrations of ethanol.
Those skilled in the art will recognize that there are two types of distillation columns, those that use trays and those that use column packing, and two operating modes of distillation columns (batch and continuous).